A screw machine is an automated machine tool for machining a wide variety of parts from bar-stock. The bar-stock may have round, square or polygonal transverse cross-sections, and may be supplied to the screw machine with initial axial lengths on the order of ten to twelve feet. The screw machine typically has a head rotatably mounted on a supporting frame. The head is controllably indexable through a plurality of successive angular positions. A plurality of circularly-spaced spindle assemblies are mounted on the head for rotation with the head about the head axis, and for rotation relative to the head about the various individual spindle assembly axes. Lengths of bar-stock are supplied axially to each spindle assembly, and rotate with the head assembly about the head axis. Hence, the rotatable head and the bar-stock supplied to the various spindle assemblies mounted on the head somewhat resembles a Gatling gun in outward appearance. The screw machine is adapted to perform a plurality of machining operations on cantilevered lengths of bar-stock that are held by and extend beyond the spindle assemblies at the various angular positions of the head.
Each spindle assembly typically has a rotatable outer spindle mounded on the head, and is elongated along a spindle axis that is parallel to the head axis. Each outer spindle has an inwardly- and forwardly-facing frusto-conical cam surface. An inner spindle is arranged within the outer spindle for rotation with the outer spindle about the spindle assembly axis, and is mounted for limited axial movement relative to the associated outer spindle. A spindle-moving means or mechanism is operatively arranged to exert a force on the inner spindle to controllably move the inner spindle in one axial direction relative to the outer spindle. A collet is mounted on the inner spindle for movement therewith. The collet has a plurality of fingers that extend forwardly from a body. These fingers have angularly-segmented outwardly- and rearwardly-facing frusto-conical cam surfaces that engage the inwardly-facing cam surface on the associated outer spindle. Each finger has a pad that is adapted to be moved radially inwardly to engage a penetrant portion of the bar-stock when the inner spindle is moved in such one axial direction relative to the outer spindle. When the spindle-moving mechanism releases the force on the inner spindle, the inner spindle moves in the opposite axial direction relative to the outer spindle, and the collet fingers are permitted to move radially outwardly (i.e., to spring back toward their original positions) such that the collet pads will to disengage from and release the bar-stock.
A feed tube is arranged within the inner spindle for rotation within the inner spindle about the spindle assembly axis, and for axial movement relative thereto. The feed tube has a plurality of feed fingers that are adapted to engage a length of penetrant bar-stock within the feed tube. A feed tube moving means or mechanism is provided for selectively causing the feed tube and feed fingers to selectively reciprocate in either axial direction (i.e., forwardly and rearwardly) relative to the inner spindle. Such movement of the feed tube is coordinated with operation of the collet so that the feed fingers may advance the bar-stock forwardly toward and through the collet when the collet fingers have moved radially away from the workpiece such that the collet is open. When the collet is closed, the feed tube is moved rearwardly away from the collet, with the feed fingers sliding along the bar-stock held in the closed collet.
One particular type of screw machine is the Davenport® five-spindle screw machine. Davenport® is a registered trademark of Brinkman Products, Inc., 167 Ames Street, Rochester, N.Y. 14611, and the parent company of Davenport Machine, Inc. of the same address. The original Davenport® machines were developed in the late 19th Century and early 20th Century. These machines had a number of cutting and forming tools mounted on the frame and operatively arranged to engage the cantilevered lengths of bar-stock held by and extending beyond the collets of the spindle assemblies. Over the years, the performance of the early Davenport® machines has been improved by the addition of servo control, man-machine interface control, improved heads, high precision heads, and the like. Many early-version and remanufactured Davenport® machines are still in service today.
The spindle assemblies of such Davenport® machines are removably mounted on the head so that they can be repaired or replaced, as necessary. The spindle assemblies of the early Davenport® machines were sized to handle ⅝-inch diameter round bar-stock, and polygonal bar-stock that fit closely within the outer transverse profile of such ⅝-inch diameter round bar-stock. This was the “regular” capacity of the early Davenport® spindle assemblies. Later models had “oversized” spindle assemblies that could handle 13/16-inch diameter round bar-stock, and polygonal bar stock that fit closely within the outer transverse profile of such 13/16-inch round bar-stock. To do this, the radial thicknesses of the various spindle assembly components (i.e., the outer spindle, the inner spindle and collet, the feed tube and feed fingers, etc.) were “thinned out” in a radial direction to accommodate the larger size of such 13/16-inch round bar-stock. These larger-capacity spindle assemblies were known as the “oversized” Davenport® spindle assemblies. However, both versions still had to fit within the original bores provided in the head. Hence, the outer envelopes of the spindle assemblies had to be the same to fit within the standard head openings.
In both the “regular” and “oversized” spindle assembly versions, the collet seat angle (i.e., the included angle of the facing cam surfaces with respect to the longitudinal axis of the spindle assembly) was about 29.5°. This worked reasonably well for softer bar-stock materials (e.g., bronzes, aluminum, some steels, and the like). In other words, the 29.5° collet seat angle was reasonably adequate to hold the cantilevered bar-stock that extended axially beyond the collet (i.e., the workpiece) against rotational and axial movement when the workpiece was subjected to the machining operations at the various angular positions of the head. This was true for both the “regular” and “oversized” spindle versions, even though the latter had been radially “thinned out” to accommodate the larger-diameter bar-stock. Even the “oversized” version generally had sufficient strength and rigidity to permit machining of cantilevered workpieces when used with such softer 13/16-inch round bar-stock materials.
However, bar-stock of harder materials was heretofore considered to be too difficult to machine on a screw machine. The principal reason for this was that the 29.5° collet seat angle was insufficient to adequately hold the cantilevered workpiece in the collet against rotational and axial movement relative thereto when such harder materials were subjected to increased forces attributable to the machining operations. This problem was exacerbated with the “oversized” spindle assemblies, in which the internal components were radially “thinned out”, because the “oversized”version lacked sufficient strength and rigidity to resist the greater forces attributable to such machining operations.
The American Iron & Steel Institute (“AISI”), of 25 Massachusetts Avenue, Suite 800, Washington, D.C. 20001, publishes a “machinability rating” for many engineering materials. This rating is derived from a series of mechanical tests, and compares the ease at which a particular material machines to an arbitrary standard benchmark under identical machining conditions. The “machinability rating” is determined by measuring the weighted averages of normal cutting speed, surface finish, and tool life for each material, and is affected by the physical properties (e.g., yield strength, abrasiveness, hardness, chemistry, microstructure, etc.) of the material. AISI has chosen B-1112 steel @ 160 Brinell hardness as that benchmark, and has assigned it a “machinability rating” of 100%.
Materials that have an AISI “machinability rating” of less than 30% have generally been considered as unsuitable for production use on a Davenport® multi-spindle screw machine. Examples of various metals and their respective “machinability ratings” are provided in the following table:
AISI “Machinability Rating”(% rel. speed based on B-Grade/AlloyApprox. Surf. Ft./Min.112 as 100%)C-1018130 78C-1117150 91C-1144125 7630270403106036316603633045204318048Nickel 200110 66Inconel ® 6005022Inconel ® 7182012Waspalloy ®4520Hastelloy ® C4018Hasteloy ® X4520[Inconel ® is a registered trademark of Huntington Alloys Corp., 3200 Riverside Drive, Huntington, W.Va. 25705. Hastelloy ® is a registered trademark of Hayes International, Inc., P.O. Box 9013, 1020 West Park Avenue, Kokomo, Ind. 46904. Waspaloy ® is reportedly a registered trademark of United Technologies Corp.]
The frequent tool changes, adjustments, and general difficulty in keeping sharp tools capable of producing the tight tolerances and fine finishes expected of these machines, negates the benefit of high-volume machining. As the “machinability rating” goes down, the tooling forces generated by the cutting tools generally go up. The problem becomes more pronounced when low-machinability materials are combined with dimensionally-larger workpieces. The forces to hold the larger workpiece under the large cutting forces generated by larger tools becomes greater than the standard work-holding collets can provide.
Around 1985, Davenport Machine, Inc. attempted to manufacture a still-larger capacity spindle assembly that would handle one-inch round bar-stock of softer materials, and polygonal bar-stock that fit closely within the outer transverse profile of such one-inch round bar-stock. The collet seat angle remained at 29.5°, and the internal components of the spindle assembly were redimensioned to accommodate one-inch diameter round bar-stock, all while keeping the confines of the spindle assembly within the outer envelope that would still fit within the head openings. Only one such machine was built and sold commercially in the hope that it could be used to machine workpieces from one-inch round bar-stock of soft material. This machine did not work as desired, and was decommissioned after several months of attempts to make it work on such larger-diameter one-inch round bar-stock. It was unable to hold machining tolerances like those of the original “regular” and “oversized” capacity Davenport® machines. The reason for the failure is thought to have been attributable to the increased machining forces that were exerted during the machining operations on the cantilevered one-inch diameter workpieces, the inability to adequately hold the one-inch bar-stock in the collet, and the reduced structural strength and rigidity of the various spindle components to resist increased machining forces on the workpiece.
Accordingly, there is believed to be a need for an improved spindle assemblies for multi-spindle screw machines generally, and the Davenport® multi-axis screw machines in particular, that will have adequate strength and rigidity to enable the use of such screw machines: (1) with some materials heretofore thought as being too difficult to machine, (2) with bar-stock having up to and including one-inch round bar-stock, and polygonal bar-stock that fits closely within the outer transverse profile of such one-inch round bar-stock, (3) that will have inner components that are better designed to enhance rigidity and stiffness, and that are designed to reduce fatigue stresses, and (4) that will adequately hold the bar-stock within the collet against relative rotational and axial movement during machining operations on the workpiece.